Tires for heavy load

ABSTRACT

There is provided a heavy duty tire made by using as a tread rubber a rubber composition obtained by compounding 100 parts by mass of a rubber component consisting of 90-30% by mass of (a) natural rubber and 10-70% by mass of (b) a solution-polymerized styrene-butadiene copolymer rubber containing tin in at least one of a middle of a polymer molecular chain and a terminal of the molecular chain and having a bound styrene content of 28-45% by mass and a vinyl bond content in a butadiene portion of less than 30 mol % with 40-60 parts by mass in total of (c) carbon black and (d) silica, provided that an amount of (d) silica as a filler is 5-20 parts by mass. In the heavy duty tire, the resistance to uneven wear is considerably improved without damaging the heat buildup and wear resistance.

TECHNICAL FIELD

This invention relates to a heavy duty tire and more particularly to aheavy duty tire considerably improving a resistance to uneven wear.

BACKGROUND ART

For the purpose of controlling the uneven wear of the tire, there havehitherto been made various means in the design of tread groove, treadcrown form, tread pattern or the like such as defense grooves, sidegrooves and the like. However, these means have a problem that thecontrol of the uneven wear is insufficient. In general, non-uniform wearis caused in ribs, blocks and the like of the tire tread by variousfactors before the occurrence of the uneven wear in the tire.

Since such a non-uniform wear is caused by an input in a radialdirection, an input in a circumferential direction or an input of acomposite thereof with respect to a rotating direction of the tire, itis attempted to improve the resistance to uneven wear by using a rubbercomposition hardly affected by these inputs in a tread and combiningwith a tire structure.

In order to prevent the uneven wear in the tire, there is used a methodof using a rubber composition compounded with styrene-butadienecopolymer rubber in the tread to enhance hysteresis loss. However, thecompounding of the styrene-butadiene copolymer rubber deteriorates theheat buildup of the tire, so that there is particularly a restriction inthe compounding amount thereof in heavy duty tires.

Now, there is proposed an attempt that a solution-polymerizedstyrene-butadiene rubber modified at a terminal of its polymer moleculeis used to enhance dispersion of carbon black through a coupling effectof its modified terminal, whereby the self-heat generation is improvedto make the heat buildup low to establish the resistance to uneven wearand the heat buildup (e.g. see JP-A-11-217004, page 3). However, it cannot be said to be sufficient in the heavy duty tire.

Further, there is proposed a technique of establishing the resistance touneven wear and the heat buildup by a combination ofsolution-polymerized styrene-butadiene copolymer rubber, silica and asilane coupling agent (e.g. see JP-A-11-59116, page 1).

However, silica has such an effect that the heat buildup is not loweredwhile enhancing the hysteresis loss, but has a fear of lowering the wearresistance under severe use condition in the heavy duty tire. In such acompounding technique, the resistance to uneven wear and the heatbuildup are conflicting relation and it is difficult to establish themsimultaneously.

DISCLOSURE OF THE INVENTION

Under the above situation, the invention is made for solving the aboveproblems and is to provide a heavy duty tire considerably improving theresistance to uneven wear without damaging the heat buildup and wearresistance of the tire.

The inventor has found that proper amounts of a solution-polymerizedstyrene-butadiene copolymer rubber having a high-cis content, silica anda specified hydrazone compound are compounded in a rubber compositionfor a tire tread, whereby the hysteresis loss can be improved whilecontrolling the self-heat buildup of the rubber and the resistance touneven wear can be improved without deteriorating the heat buildup andthe wear resistance, and as a result, the invention has beenaccomplished.

That is, the invention provides a heavy duty tire characterized by usingas a tread rubber a rubber composition obtained by compounding 100 partsby mass of a rubber component consisting of 90-30% by mass of (a)natural rubber and 10-70% by mass of (b) a solution-polymerizedstyrene-butadiene copolymer rubber containing tin in at least one of amiddle of a polymer molecular chain and a terminal of the molecularchain and having a bound styrene content of 28-45% by mass and a vinylbond content in a butadiene portion of less than 30 mol % with 40-60parts by mass in total of (c) carbon black and (d) silica, provided thatan amount of (d) silica as a filler is 5-20 parts by mass.

Further, the invention provides a heavy duty tire in which 0.3-3.0 partsby mass of (e) a hydrazone compound is further compounded per 100 partsby mass of the rubber component.

BEST MODE FOR CARRYING OUT THE INVENTION

As the component (a) of the rubber composition used in the tireaccording to the invention is used natural rubber, but a part of naturalrubber may be replaced with polyisoprene rubber having the samestructure as the natural rubber.

The amount of (a) natural rubber is 90-30% by mass based on 100 parts bymass of the rubber component, preferably 60-40% by mass. When the amountof the component (a) exceeds 90% by mass, there is no effect on theresistance to uneven wear, while when it is less than 30% by mass, it isdifficult to maintain the low heat buildup.

In the rubber composition used in the tire according to the invention,as the component (b) is used a solution-polymerized styrene-butadienecopolymer rubber containing tin in at least one of a middle of a polymermolecular chain and a terminal of the molecular chain and having a boundstyrene content of 28-45% by mass and a vinyl bond content in abutadiene portion of less than 30 mol %.

The amount of (b) solution-polymerized styrene-butadiene copolymerrubber is 10-70% by mass based on 100 parts by mass of the rubbercomponent, preferably 40-60% by mass. When the amount of the component(b) is less than 10% by mass, the improving effect on the uneven wear ispoor, while when it exceeds 70% by mass, the fracture properties areconsiderably deteriorated.

Also, the bound styrene content in the component (b) of the rubbercomposition used in the tire according to the invention is 28-45% bymass. When the content is less than 28% by mass, the effect ofestablishing the resistance to uneven wear and the low heat buildup ispoor, while when it exceeds 45% by mass, the wear resistance isdeteriorated.

Further, the vinyl bond content in the component (b) is less than 30 mol%. When the content is not less than 30 mol %, the wear resistancelowers.

In the component (b), a molecular structure of a reaction portion, whichis formed by reacting a terminal carbanion of hydrocarbon group-bondedstyrene-butadiene copolymer obtained through an organolithium initiatorconsisting of a hydrocarbon group and lithium with a coupling agent or amodifying agent, has a tin-carbon bond.

For example, the molecular structure of the reaction portion, which isformed by a stoichiometric reaction between a one-side terminalcarbanion of a butyl group terminal bonded styrene-butadiene copolymerobtained through a butyllithium initiator consisting of butyl group as ahydrocarbon group and lithium and tin tetrachloride as a coupling agent,has a structure that four butyl terminal styrene-butadiene copolymersare bonded to tin, i.e. a structure that tin-carbon bond is existent ina middle portion of the final polymer.

When tributyltin chloride is used as a modifying agent instead of tintetrachloride in the above embodiment, there is obtained a final polymerhaving a structure of tin-carbon bond at the other terminal of the butylgroup terminal styrene-butadiene copolymer.

The above structure has an interaction with a filler such as carbonblack or the like and is essential for improving the low hysteresisproperty and the wear resistance.

Furthermore, a weight average molecular weight before the modificationreaction of the component (b) is preferably 5×10⁴-100×10⁴, morepreferably 10×10⁴-100×10⁴ for providing excellent properties, and also amolecular weight distribution before the modification reaction ispreferably 1.0-1.3.

Moreover, the production method of the component (b) is not particularlylimited as far as the molecular structure defined in the invention isobtained, but the copolymer of the component (b) is easily obtained bythe following method (a detail of which is disclosed in, for example,JP-A-9-316132). The styrene-butadiene copolymer of the component (b) isobtained by copolymerizing styrene and butadiene in a hydrocarbonsolvent in the presence of (1) an organolithium initiator consisting ofa hydrocarbon group and lithium, (2) at least one potassium compoundselected from the group consisting of potassium alcoholate, potassiumsulfonate and potassium carboxylate, and (3) at least one compoundselected from the group consisting of ether compounds and aminecompounds, and adding a tin compound as a modifying agent after thecopolymerization.

As the hydrocarbon solvent can be used an aromatic hydrocarbon solventsuch as benzene, toluene, xylene or the like; an aliphatic hydrocarbonsolvent such as n-pentane, n-hexane, n-butane or the like; an alicyclichydrocarbon solvent such as methylcyclopentane, cyclohexane or the like;and a mixture thereof, which are not particularly limited.

As the organolithium initiator, mention may be made of alkyllithiumssuch as methyllithium, ethyllithium, propyllithium, n-butyllithium,sec-butyllithium, t-butyllithium, hexyllithium, octyllithium and thelike; aryllithiums such as phenyllithium, tolyllithium and the like; andaralkyllithiums such as benzyllithium and the like.

Among them, n-butyllithium and sec-butyllithium are preferable from aviewpoint of industrial production. These organolithium initiators maybe used alone or in a combination of two or more.

The amount of such a polymerization initiator added is determined inaccordance with the desired molecular weight of the copolymer, but isusually 0.05-15 mmol, preferably 0.1-10 mmol per 100 g of the monomer.When the amount exceeds 15 mmol, it is difficult to obtain a highermolecular weight polymer, while when it is less than 0.05 mmol, thepolymerization may not proceed.

As a randomizer are used a potassium compound, an ether compound and anamine compound. The term “randomizer” used herein means a compoundhaving an action of increasing the vinyl bond content in the butadieneportion of the styrene-butadiene copolymer (this action is small in thepotassium compound), a randomization of composition distribution ofbutadiene unit and styrene unit, and the like.

The potassium compound used herein are potassium alcoholate, potassiumsulfonate and/or potassium carboxylate. As the potassium alcoholate maybe mentioned, for example, potassium t-butylate, potassium t-amylate,potassium ethylate, potassium isopropylate, potassium octylate,potassium dodecylate, potassium nonylphenylate and the like. Among them,potassium t-amylate and potassium nonylphenylate are preferable in viewof the effect.

As the potassium sulfonate may be mentioned, for example, potassiumdodecylbenzene sulfonate, potassium naphthalene sulfonate and the like.Among them, potassium dodecylbenzene sulfonate is preferable in view ofthe effect.

As the potassium carboxylate may be mentioned, for example, potassiumstearate, potassium decanoate, potassium dodecanoate, potassium octoate,potassium naphthate and the like. Among them, potassium naphthate ispreferable in view of the effect.

The amount of the potassium compound added is 0.01-0.2 mol equivalentper 1 mol of lithium, preferably 0.03-0.09 mol equivalent in view of theeffect. When the amount is less than 0.01 mol equivalent, there is noeffect as the randomizer, while when it exceeds 0.2 mol equivalent,undesirable side reaction such as metalation or the like occurs.

Also, the ether compound and/or the amine compound are used togetherwith the potassium compound. As the ether compound and/or the aminecompound, there can be used compounds usually used as a randomizer inthe copolymerization of styrene and butadiene, which are notparticularly limited. Among them, a dialkoxyalkyl compound such asdiethoxy ethane or the like; a diethylene glycol dialkyl ether compoundsuch as diethylene glycol dimethyl ether, diethylene glycol diethylether or the like; an ethylene glycol dialkyl ether compound such asethylene glycol dimethyl ether, ethylene glycol diethyl ether or thelike; a tetrahydrofuran oligomer compound such as diterahydrofurylpropane or the like; tetrahydrofuran; a diamine compound such astetramethylethylene diamine or the like; and a triamine compound such aspentamethylene diethylene triamine or the like are preferably used.

The amount of the ether compound and/or the amine compound added ispreferable to be an amount that the vinyl bond content in the butadieneportion is not more than 30 mol %. This amount is hardly identifiedbecause it is dependent upon the kind of the ether compound or the aminecompound, but is usually 0.01-2.0 mol equivalent per 1 mol of lithium.For example, a proper amount when using tetrahydrofuran is 0.5-2.0 molequivalent, and a proper amount when using diethylene glycol dimethylether is 0.03-0.1 mol equivalent.

Also, the coupling agent or modifying agent is a tin compound. As thetin compound are mentioned, for example, a tin halide such as tintetrachloride or the like; and an organic tin chloride compound such asbutyltin trichloride, dibutyltin dichloride, dioctyltin dichloride,diphenyltin dichloride, tributyltin chloride, triphenyltin chloride orthe like. It can be used in such an amount that an active terminallithium of the styrene-butadiene copolymer is equivalent to a halogenatom of the above compound.

Among these compounds, tin tetrachloride, the organic tin dichloride orthe like contributes to a low hysteresis loss because it is subjected toa coupling reaction with the active terminal of the styrene-butadienecopolymer to provide a coupled copolymer and the coupling efficiency isnot less than 60%. Also, the organic tin monochloride compound improvesthe low hysteresis loss because it is reacted with the active terminallithium to provide a terminal-modified copolymer.

In the rubber composition used in the tire according to the invention,carbon black (c) and silica (d) are used as a filler. By compoundingcarbon black can be obtained an effect of improving the wear resistanceleading to the improvement of the fracture resistance. The compoundingamount in total of carbon black (c) and silica (d) is 40-60 parts bymass based on 100 parts by mass of the rubber component. When it is lessthan 40 parts by mass, the wear resistance is deteriorated, while whenit exceeds 60 parts by mass, the low heat buildup is deteriorated.

The kind of carbon black (c) is not particularly limited, but can beused by arbitrarily selecting from those usually used as a filler forthe reinforcement of the conventional rubber. As the carbon black (c)are mentioned, for example, FEF, SRF, HAF, ISAF, SAF and the like. Amongthem, carbon black SAF is preferable.

Furthermore, the compounding amount of silica as the component (d) 5-20parts by mass based on 100 parts by mass of the rubber component. Whenthe amount is less than 5 parts by mass, the effect of improving theresistance to uneven wear is poor, while when it exceeds 20 parts bymass, the wear resistance and the fracture properties are largelydeteriorated. From the same viewpoint, it is preferably 7-15 parts bymass. Also, silica is not particularly limited, but can be used byproperly selecting from those usually used as a filler for thereinforcement of the conventional rubber.

For example, there are mentioned wet silica (hydrated silica), drysilica (anhydrated silica), calcium silicate, aluminum silicate and thelike. Among them, wet silica is preferable.

Further, silica of the component (d) is preferable to have a nitrogenadsorption specific surface area (N₂SA) of 120-240 m²/g. When N₂SA isless than 120 m²/g, the wear resistance becomes insufficient, while whenN₂SA exceeds 240 m²/g, the poor dispersion is caused, which results inthe considerable lowering of the low heat buildup, wear resistance andfactory operability.

Moreover, the above N₂SA is a value measured according to ASTM D4820-93after the drying at 300° C. for 1 hour.

In the rubber composition used in the tire according to the invention, ahydorazone compound (e) is compounded in an amount of 0.3-3.0 parts bymass based on 100 parts by mass of the rubber component.

When the amount of the component (e) is not less than 0.3 part by mass,the effect of improving the resistance to uneven wear and the heatbuildup is obtained, while when it is not more than 3.0 parts by mass,there is no problem in the factory operability.

As the hydorazone compound used as the component (e) in the rubbercomposition used in the tire according to the invention, naphthoic acidhydrazide and salicylic acid hydrazide represented, for example, by thefollowing general formulae (I), (II) are preferable in view ofperformances.

(wherein R¹ and R² are a hydrogen atom or a hydrocarbyl group having acarbon number of 1-18, respectively and may be the same or different, orR¹ and R² may be bonded to each other to form a ring structure. As thehydrocarbyl group having a carbon number of 1-18 may be mentioned astraight or branched alkyl group having a carbon number of 1-18, astraight or branched alkenyl group having a carbon number of 2-18, acycloalkyl group having a carbon number of 3-8, an aryl group having acarbon number of 6-18 and an aralkyl group having a carbon number of7-18. On a ring of the cycloalkyl group, the aryl group or the aralkylgroup may be provided a proper substituent such as a lower alkyl group,a lower alkoxyl group, an amino group, an alkyl-substituted amino group,a hydroxyl group or the like.)

As the hydrazide compounds of the general formulae (I) and (II), thereare concretely 2-hydroxy-N′-(1-methylethylidene)-3-naphthoic acidhydrazide, 2-hydroxy-N′-(1-methylpropylidene)-3-naphthoic acidhydrazide, 2-hydroxy-N′-(1-methylbutylidene)-3-naphthoic acid hydrazide,2-hydroxy-N′-(1,3-dimethylbutylidene)-3-naphthoic acid hydrazide,2-hydroxy-N′-(2,6-dimethyl-4-heptylidene)-3-naphthoic acid hydrazide,N′-(1-methylethylidene)-salicylic acid hydrazide,N′-(1-methylpropylidene)-salicylic acid hydrazide,N′-(1-methylbutylidene)-salicylic acid hydrazide,N′-(1,3-dimethylbutylidene)-salicylic acid hydrazide, andN′-(2,6-dimethyl-4-heptylidene)-salicylic acid hydrazide.

Among them, 2-hydroxy-N′-(1,3-dimethylbutylidene)-3-naphthoic acidhydrazide (BMH) is particularly preferable.

The hydrazone compound of the component (e) has an action of suppressingthe lowering of elastic modulus resulted from the over vulcanization dueto reversion of natural rubber (a) to control the lowering of the lowheat buildup and the wear resistance.

By compounding the hydrazone compound (e) is raised the modulus ofelasticity at a low strain region to suppress the deformation of thetread rubber, whereby the lowering of the hysteresis loss preventedwhile lowering the modulus of elasticity at a high strain region by theaction of silica to ensure the elongation of the tread rubber and hencethe resistance to uneven wear and the low heat buildup can beestablished simultaneously.

Also, the rubber composition used in the tire according to the inventionmay be properly compounded with the other additives such as sulfur,vulcanization accelerator, process oil, antioxidant and the like, ifnecessary.

The rubber composition used in the tire according to the invention isobtained by milling with a milling machine such as rolls, internal mixeror the like, and is suitably used as a rubber composition for a tread ofa heavy duty tire highly balancing the resistance to uneven wear, wearresistance and low heat buildup.

The tire according to the invention is produced by a usual method usingthe above rubber composition.

That is, the rubber composition formed by compounding the above variousadditives, if necessary, is extrusion-molded into various members fortire at an unvulcanized state, which are attached on a tire buildingmachine in the usual manner to form a green tire. The green tire isheated and pressurized in a vulcanization machine to obtain a tire.

Moreover, as a gas filled in the tire can be used normal air or airhaving a changed oxygen partial pressure, and an inert gas such asnitrogen or the like.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES

Various measurements are carried out by the following methods.

Evaluations of green rubber and vulcanized rubber Vinyl bond content inbutadiene portion (mol %): is measured by an infrared spectrophotometry(Morero's method).

Bound styrene content: is measured from an absorption intensity ofaromatic proton in a nuclear magnetic resonance (NMR) spectrum.

Evaluation of Test Tire

Evaluation of heat buildup: A drum test is carried out under conditionsof constant speed and step load to measure a temperature at a positionof a constant depth inside the tire, which is represented by an index onthe basis that a value of control (Comparative Example 1) is 100. Thelarger the index value, the larger the effect of lowering heat buildup.

Evaluation of wear resistance: A wear resistance of a tread rubber inthe tire after the running over 100000 km is calculated by an equationof [running distance/(groove depth before running−groove depth afterrunning)] and represented by an index on the basis that a value ofComparative Example 1 is 100. The larger the index value, the better thewear resistance.

Evaluation of resistance to uneven wear: The tire is mounted on a frontwheel of a truck and run over 100000 km, and thereafter uneven worn areais measured and a reciprocal number of the measured value is representedby an index on the basis that a value of Comparative Example 1 is 100.The larger the index value, the better the resistance to uneven wear.

Production Example 1 Polymer A

Into a glass pressure container of 800 ml dried and purged with nitrogenare charged 300 g of cyclohexane, 32.5 g of 1,3-butadiene monomer, 17.5g of styrene monomer, 0.025 mmol of potassium t-amylate and 1 mmol ofTHF, and 0.45 mmol of n-butyllithium (BuLi) is added to conductpolymerization at 50° C. for 3 hours. The polymerization system isuniform and transparent without precipitation from a start ofpolymerization to an end thereof. The polymerization conversion isapproximately 100%.

To this polymerization system is added 0.12 mmol of 1M cyclohexanesolution of DOTDC (dioctyltin dichloride) as a modifying agent toconduct modification reaction for 30 minutes. To the polymerizationsystem is further added 0.5 ml of 5% isopropanol solution of2,6-di-tertiary butyl paracresol (BHT) to stop the reaction, which isfurther dried to obtain a polymer A. The bound styrene content is 35% bymass and the vinyl bond content is 23 mol %.

Production Example 2 Polymer B

Into a glass pressure container of 800 ml dried and purged with nitrogenare charged 300 g of cyclohexane, 37.5 g of 1,3-butadiene monomer, 12.5g of styrene monomer, 0.03 mmol of potassium t-amylate and 2 mmol ofTHF, and further added with 0.41 mmol of hexamethylene imine as asecondary amine. It is further added with 0.45 mmol of n-butyllithium(BuLi) to conduct polymerization at 50° C. for 2.5 hours. Thepolymerization system is uniform and transparent without precipitationfrom a start of polymerization to an end thereof. The polymerizationconversion is approximately 100%.

To this polymerization system is added 0.09 mmol of 1M cyclohexanesolution of TTC (tin tetrachloride) as a modifying agent to conductmodification reaction for 30 minutes. To the polymerization system isfurther added 0.5 ml of 5% isopropanol solution of 2,6-di-tertiary butylparacresol (BHT) to stop the reaction, which is further dried to obtaina polymer B. The bound styrene content is 25% by mass and the vinyl bondcontent is 28 mol %.

Production Example 3 Polymer C

Into a glass pressure container of 800 ml dried and purged with nitrogenare charged 300 g of cyclohexane, 32.5 g of 1,3-butadiene monomer, 17.5g of styrene monomer, 0.025 mmol of potassium t-amylate and 1 mmol ofTHF, and 0.45 mmol of n-butyllithium (BuLi) is added and further 0.41mmol of hexamethylene imine is added as a secondary amine to conductpolymerization at 50° C. for 3 hours. The polymerization system isuniform and transparent without precipitation from a start ofpolymerization to an end thereof. The polymerization conversion isapproximately 100%.

To this polymerization system is added 0.12 mmol of 1M cyclohexanesolution of DOTDC (dioctyltin dichloride) as a modifying agent toconduct modification reaction for 30 minutes. To the polymerizationsystem is further added 0.5 ml of 5% isopropanol solution of2,6-di-tertiary butyl paracresol (BHT) to stop the reaction, which isfurther dried to obtain a polymer C. The bound styrene content is 35% bymass and the vinyl bond content is 24 mol %.

The evaluation results are shown in Table 1, and characteristic valuesof the polymers A-C are shown in Table 2. TABLE 1 Compar- Compar-Compar- Compar- ative ative ative ative Example Example Example ExampleExample Example Example 1 Example 2 Example 3 Example 4 1 2 3 4 5 6Natural rubber 50 50 40 50 50 50 40 50 50 50 Polymer A *1 — — — — 50 5060 — — 50 Polymer B *2 50 50 60 50 — — — — — — Polymer C *3 — — — — — —— 50 50 — Carbon black *4 45 40 45 40 40 40 40 40 40 45 Silica *5 — 10 —4 10 10 10 10 15 10 Antioxidant 6C *6 1 1 1 1 1 1 1 1 1 1 Hydrazonecompound *7 — — — — — 1 1 1 1 1 Stearic acid 2 2 2 2 2 2 2 2 2 2 Zincwhite 3 3 3 3 3 3 3 3 3 3 Vulcanization accelerator 1.2 1.2 1.2 1.2 1.21.2 1.2 1.2 1.2 1.2 CZ *8 Sulfur 1 1 1 1 1 1 1 1 1 1 Bound styrenecontent 25 25 25 25 35 35 35 35 35 35 Vinyl bond content 28 28 28 28 2323 23 24 24 23 Heat buildup 100 95 94 104 103 108 102 113 113 100Resistance to uneven wear 100 109 112 102 120 128 136 128 146 138 Wearresistance 100 100 100 95 105 105 100 100 102 106*1. Polymer A: Production Example 1, bound styrene content 35% by mass,vinyl bond content 23 mol%*2. Polymer B: Production Example 2, bound styrene content 25% by mass,vinyl bond content 28 mol%*3. Polymer C: Production Example 3, bound styrene content 35% by mass,vinyl bond content 24 mol%*4. carbon black: Seaste 9, trade mark, made by Tokai Carbon Co., Ltd.*5. silica: Nipsil AQ, trade mark, made by Nippon Silica Co., Ltd. N₂SA(200 m²/g)*6. antioxidant 6C: N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylene diamine*7. hydrazone compound:2-hydroxy-N′-(1,3-dimethylbutylidene)-3-naphthoic acid hydrazide (BMH)*8. vulcanization accelerator CZ: N-cyclohexyl-2-benzothiazylsulfenamideNote) As to numerical vales in Table 1, the values of natural rubber andpolymers A-C are mass%, and the values of carbon black, silica,antioxidant 6C, hydrazone compound, stearic acid, zinc white,vulcanization accelerator CZ and sulfur are parts by mass based on 100parts by mass of the rubber component.

TABLE 2 Bound styrene content Vinyl bond content (weight %) (mol %)Polymer A 35 23 Polymer B 25 28 Polymer C 35 24

Examples 1-6, Comparative Examples 1-4

Each composition having a compounding recipe shown in Table 1 is milledby using a Banbury mixer. The resulting rubber composition is used as atread rubber to prepare a tire having a tire size of 295/75 R22.5 andthen the heat buildup, wear resistance and resistance to uneven wear areevaluated by the aforementioned methods.

As seen from the above results, in the heavy duty tires according to theinvention, the resistance to uneven wear is largely improved whilemaintaining and improving the heat buildup and the wear resistance.

INDUSTRIAL APPLICABILITY

The invention can provide a heavy duty tire considerably improving theresistance to uneven wear without sacrificing the heat buildup and wearresistance.

1. A heavy duty tire characterized by using as a tread rubber a rubbercomposition obtained by compounding 100 parts by mass of a rubbercomponent consisting of 90-30% by mass of (a) natural rubber and 10-70%by mass of (b) a solution-polymerized styrene-butadiene copolymer rubbercontaining tin in at least one of a middle of a polymer molecular chainand a terminal of the molecular chain and having a bound styrene contentof 28-45% by mass and a vinyl bond content in a butadiene portion ofless than 30 mol % with 40-60 parts by mass in total of (c) carbon blackand (d) silica, provided that an amount of (d) silica as a filler is5-20 parts by mass.
 2. A heavy duty tire according to claim 1, wherein0.3-3.0 parts by mass of (e) a hydorazone compound is further compoundedper 100 parts by mass of the rubber component.
 3. A heavy duty tireaccording to claim 2, wherein the hydrazone compound (e) is2-hydroxy-N′-(1,3-dimethylbutylidene)-3-naphthoic acid hydrazide.